HSC Exam Practice

Sample response. A unicellular organism consists of a single cell that performs all life functions independently with no division of labour. A colonial organism consists of a group of genetically identical cells living together, where cells may show limited division of labour (e.g. Volvox gonidia vs somatic cells) but each cell retains the ability to survive independently if separated from the colony.

Marking notes. 1 mark for unicellular defined correctly (single cell, fully independent, no DoL); 1 mark for colonial defined correctly (group of genetically identical cells, cells retain independence); 1 mark for correctly comparing division of labour between the two (none in unicellular; limited/partial in colonial).

Sample response. The three requirements are: (1) cell adhesion, cells must physically stick together; (2) cell communication, cells must be able to signal each other (via hormones, nerves or cytoplasmic junctions); and (3) cell differentiation, cells must permanently specialise through selective gene expression.

Marking notes. 1 mark per correct requirement (adhesion, communication, differentiation). Accept reasonable synonyms (e.g. “cell sticking” for adhesion, “cell signalling” for communication). All three must be named for full marks.

Sample response. Cell specialisation is the permanent modification of a cell's structure to optimise it for a specific function. During differentiation, the red blood cell permanently expels its nucleus. This is advantageous because the nucleus is a large organelle that would occupy internal volume; by removing it, the cell maximises the space available for haemoglobin, dramatically increasing its oxygen-carrying capacity. The biconcave disc shape that results also increases the SA:V ratio, improving the rate of gas exchange across the cell membrane.

Marking notes. 1 mark for correctly defining or applying cell specialisation (permanent structural modification for a function); 1 mark for explaining that nucleus removal maximises haemoglobin volume (the structural basis of the advantage); 1 mark for linking this to an enhanced functional consequence (increased O₂ transport capacity or improved gas exchange).

Sample response. Each level of the hierarchy is built from the level below and performs functions that the level below cannot achieve alone. For example, no single cardiac muscle cell can pump blood around the body, but cardiac muscle cells form cardiac muscle tissue, which forms the heart (an organ), which drives the cardiovascular system, which sustains the whole organism. A single cell cannot circulate blood, but the organ built from millions of coordinated cells can. This is the functional consequence of hierarchical organisation: emergent capability at each higher level.

Marking notes. 1 mark for stating the key principle (each level enables functions the level below cannot perform alone); 1 mark for a correct and specific example of this principle (the example must span at least two levels of the hierarchy); 1 mark for explicitly linking the structural change at one level to the new functional capability at the next (e.g. cells → tissue → organ allows pumping that cells alone cannot perform).

Sample response. As cell width increases, the SA:V ratio decreases. The decrease is steep at small widths (from 6.00 to 0.60 between 1 and 10 µm) and becomes more gradual at larger widths, producing a curve that flattens toward zero.

Marking notes. 1 mark for stating that SA:V ratio decreases as cell width increases (non-linear/curvilinear decrease is a bonus; accept “decreasing” without shape description for 1 mark).

Sample response. Percentage decrease = [(6.00 − 0.60) / 6.00] × 100 = [5.40 / 6.00] × 100 = 90%.

Marking notes. 1 mark for correct working (subtracting, dividing by original, multiplying by 100); 1 mark for correct final answer of 90%.

Sample response. All exchange of nutrients, gases and waste in a cell must occur across the cell membrane (the surface). As a unicellular organism grows larger, its volume increases proportionally faster than its surface area (SA scales with the square of size; V scales with the cube), so the SA:V ratio falls [1]. A very large unicellular cell cannot supply its interior by diffusion, the centre becomes starved of oxygen and nutrients and waste accumulates, limiting the maximum viable cell size [1]. Multicellular organisms overcome this by keeping each individual cell small (maintaining a high SA:V ratio for efficient exchange per cell), while using specialised internal transport systems (e.g. the cardiovascular system in animals, vascular tissue in plants) to distribute materials to all cells throughout a large body [1].

Marking notes. 1 mark for explaining why large cells have a low SA:V ratio (SA scales as square, V as cube of size) and why this limits exchange; 1 mark for linking low SA:V to the functional limit (diffusion insufficient → cell interior deprived); 1 mark for explaining the multicellular solution (cells remain small + specialised transport systems supply the whole body).

Sample response. Both Volvox and a human are eukaryotic organisms whose cells contain membrane-bound organelles including a nucleus, mitochondria and ribosomes [similarity, 1 mark]. However, whereas Volvox is a colony of up to 50,000 genetically identical cells showing only limited division of labour (somatic cells for movement/photosynthesis; gonidia for reproduction), a human consists of hundreds of permanently distinct cell types (e.g. neurons, red blood cells, muscle cells) with complete and irreversible division of labour [difference 1, 1 mark]. In terms of cell independence, Volvox somatic cells can survive and form new colonies if isolated from the colony, whereas human cells such as cardiac muscle cells or neurons die rapidly when separated from the organism because they depend entirely on the cardiovascular and nervous systems for oxygen, glucose and regulatory signals [difference 2, 1 mark]. This permanent inability to survive alone is called interdependence, and it is the critical boundary between colonial and multicellular organisation. Because the majority of Volvox cells retain viability when isolated, the criterion of permanent interdependence is not met, and Volvox remains classified as colonial [permanent interdependence justification, 1 mark].

Marking notes. 1 mark for at least one correct similarity (both eukaryotic; both contain cells with nucleus and organelles; both perform photosynthesis or metabolic functions). 1 mark for first difference (e.g. limited DoL in Volvox vs complete, permanent DoL in humans / cell specialisation reversible vs irreversible). 1 mark for second difference (e.g. Volvox cells survive isolation vs human cells cannot / partial vs permanent interdependence). 1 mark for defining permanent interdependence and using it to justify colonial classification of Volvox (must explicitly state that Volvox cells can survive isolation → permanent interdependence not met).

Sample Band 6 response. Multicellular organisation confers several structural advantages over unicellular life, each enabling functional consequences unavailable to a single cell.

First, division of labour is the allocation of permanently different roles to different cells. Red blood cells permanently expel their nucleus and pack with haemoglobin, maximising oxygen-carrying capacity, a unicellular organism that also had to replicate DNA, synthesise proteins and perform all other life functions could not afford this structural sacrifice. The same principle applies to the palisade mesophyll cell, which packs chloroplasts at the top of the leaf purely for photosynthesis.

Second, multicellular organisms overcome the SA:V constraint by keeping individual cells small (ensuring efficient diffusion per cell) while distributing materials via internal transport systems (cardiovascular system, phloem/xylem). A unicellular organism must rely on diffusion across a single cell membrane, limiting viable body size to the microscopic scale. Multicellular organisms can reach sizes from millimetres to metres.

Third, stem cells enable ongoing repair and renewal. Stem cells continuously replace worn-out or damaged specialised cells (e.g. intestinal epithelium replaced every 3–5 days; red blood cells replaced every 120 days). A unicellular organism has no repair mechanism, damage means death. This advantage allows multicellular organisms to live for years, decades or centuries, whereas individual unicellular organisms typically survive hours to days.

Fourth, hierarchical organisation (organelle → cell → tissue → organ → system → organism) allows the emergence of complex structures (eyes, brains, immune systems) impossible for a single cell. A neuron alone cannot process information; a neural network in the brain integrates millions of signals per second. Each level of hierarchy unlocks capabilities that the level below cannot achieve.

In summary, multicellular organisation justifies itself through structural advantages, specialisation, transport, repair and hierarchy, each producing functional consequences that would be physically impossible for a single cell to achieve.

Marking criteria.

• 1 markStates an overall justification claim (multicellularity is advantageous; links to structural basis).
• 1 markAdvantage 1 (division of labour): structural basis (permanent specialisation / cells optimised for one role) + a consequence impossible for unicellular (e.g. red blood cell discards nucleus to maximise haemoglobin).
• 1 markAdvantage 2 (SA:V + transport): structural basis (cells stay small; internal transport distributes resources) + consequence impossible for unicellular (large body size possible).
• 1 markAdvantage 3 (stem cell repair): structural basis (stem cells replace damaged/lost cells) + consequence (longevity; damage does not kill the whole organism; unicellular has no equivalent).
• 1 markAdvantage 4 (hierarchy/complex structures): structural basis (hierarchical organisation: cells → tissues → organs → systems) + consequence (complex structures, e.g. brain, immune system, eye, impossible at cell level).
• 1 markEach advantage is explicitly linked to a structural feature of multicellularity and a functional consequence unavailable to a unicellular organism (this mark is awarded when at least three advantages have both structural basis AND explicit unicellular contrast).
• 1 markJustified conclusion using precise lesson terminology (division of labour, interdependence, specialisation, hierarchy, cell differentiation).